Method and a device for recovering metals comprising dissolving, concentrating and electrowinning means with rotatable cathode

ABSTRACT

A method and a device for recovering metals from a metal-containing flow, wherein: a) the metal-containing flow ( 7 ) and a solvent ( 8 ) are supplied to a dissolving unit ( 1 ), whereby a metal-containing solution is formed; b) the metal-containing solution is then supplied to a concentration unit ( 2 ); c) the metal-containing solution is separated in the concentration unit into a small-volume flow ( 14 ) containing a high concentration of metal salts and/or metal hydroxides, and a large-volume flow ( 13 ) containing a low concentration of metal salts and/or metal hydroxides; d) the small-volume flow ( 14 ) containing a high concentration of metal salts and/or metal hydroxides is supplied to an electrochemical unit ( 5 ); and e) the small-volume flow containing a high concentration of metal salts and/or metal hydroxides is separated in said electrochemical unit into a flow ( 16 ) containing one or more metals, and a flow ( 15 ) containing a low concentration of metal salts and/or metal hydroxides.

This application is a 371 of PCT/NL98/00349 filed Jun. 17, 1998.

The invention relates to a method and a device for recovering metalsfrom a metal-containing flow, for example from ores, metal-containingresidues and waste materials.

With known methods and devices for recovering metals frommetal-containing flows, a great of energy is consumed. Saidmetal-containing flows may be flows of solid materials, for exampleores, or of liquid materials, for example metal-containing slurries orsolutions. Since the energy is usually generated by the combustion offossil fuels, large amounts of gases, such as carbon dioxide, areemitted. The emission of such “greenhouse” gases affects theenvironment. Moreover, due to the large energy consumption, such methodsand devices are not very efficient.

The invention provides a solution for the above problem. Accordingly,the invention relates to a method as referred to in the introduction,wherein:

a. the metal-containing flow and a solvent are supplied to a dissolvingunit, whereby a metal-containing solution is formed;

b. the metal-containing solution is then supplied to a concentrationunit;

c. the metal-containing solution is separated in the concentration unitinto a small-volume flow containing a high concentration of metal saltsand/or metal hydroxides, and a large-volume flow containing a lowconcentration of metal salts and/or metal hydroxides;

d. the small-volume flow containing a high concentration of metal saltsand/or metal hydroxides is supplied to an electrochemical unit; and

e. the small-volume flow containing a high concentration of metal saltsand/or metal hydroxides is separated in said electrochemical unit into aflow containing one or more metals, and a flow containing a lowconcentration of metal salts and/or metal hydroxides.

The term metal salts and/or metal hydroxides is understood to includeinorganic and metallo-organic compounds containing metal cations,wherein said compounds may be single salts (for example nickel (II)chloride) or complex salts (that is, salts which contain the same metalsexhibiting different stages of oxidation, or different metals, which mayor may not exhibit the same stage of oxidation, or single hydroxides(for example tin hydroxide) or complex hydroxides (for example borates),and wherein one or more ligands, for example ammonia, may bonded to themetal cations.

The advantages of the invention are that the method requires much lessenergy than usual, that the method proceeds quickly and that it ispossible to use devices of much smaller dimensions than those which areused with methods according to the prior art. These advantages areachieved in particular because separate circuits are used for thedissolving method and for the electrochemical method. Furthermore it ispossible to carry out parts of the methods or the entire method as suchfor recovering all types of metals, in particular the metals copper,lead, tin, zinc, antimony, chromium, gold, cadmium, silver and nickel,and alloys such as brass. Furthermore it is possible to adapt methodparameters such as the flow rate, the temperature and the like to thedemand and to the chemical and/or physical requirements which are madeof the raw material and of the final product.

In the dissolving unit, a metal-containing flow, for example scrap,sludge and/or ash, is mixed with one or more solvents, for examplewater. The solvents may contain additives, which promote the dissolutionof the metals. Examples of such additives are acids and bases. When themetals are being dissolved, a solution of metal salts and/or metalhydroxides is formed, as well as a vapour and gas flow, whichsubstantially contains solvent in the vapour phase, for example watervapour, and reaction gases, for example hydrogen, carbon dioxide,ammonia, oxygen, nitrogen and nitrogen oxides.

The separation of the solution in the concentration unit preferablytakes place by cooling down the solution, whereby a heat-containing flowand a cooled-down solution are formed. Part of the heat which iscontained in the original solution is transferred to the heat-containingflow as a result of the evaporation of volatile components which arepresent in the solution, so that the original solution is cooled down.The heat is preferably transferred as a result of the evaporation of thesolvent or the solvents, for example water or a mixture of water and oneor more other solvents, from the solution. Accordingly, theheat-containing flow substantially consists of a vapour-phase solvent orsolvents from the solution, and in particular substantially of watervapour.

The heat-containing flow is preferably led to a heat exchange plant, sothat the heat of the heat-containing flow can be transferred to anotherprocess flow, which is to be heated. Consequently, the heat is largelywithdrawn from the heat-containing flow, so that the vapour-phasesolvent or solvents will condense and a relatively cold flow consistingof a solvent or solvents is formed. Said relatively cold flow consistingof a solvent or solvents can then be led back to the concentration unit.

When the solution is cooled down to a temperature below the saturationtemperature, metal salts and/or metal hydroxides will separate from thesolution, for example by precipitating or crystallizing. According tothe invention the cooled-down solution is thus carried to a settlingunit, where the cooled-down solution is separated into a small-volumeflow containing a high concentration of metal salts and/or metalhydroxides, and a large-volume flow containing a low concentration ofmetal salts and/or metal hydroxides.

The large-volume flow, which contains a low concentration of metal saltsand/or metal hydroxides, can be led back again, for example to thedissolving unit or to the concentration unit. According to theinvention, this large-volume flow, which contains a low concentration ofmetal salts and/or metal hydroxides, is preferably led back to thedissolving unit.

The large-volume flow containing the low concentration of metal saltsand/or metal hydroxides is preferably heated before being led back tothe dissolving unit or to the concentration unit. Preferably, the heatwhich can be transferred via the heat-containing flow and the heatexchange plant is used for heating said large-volume flow. A majoradvantage of this embodiment is the fact that the energy requirement ofthis method is much lower than that of comparable, conventional methods.

The small-volume flow containing a high concentration of metal saltsand/or metal hydroxides is fed to an electrochemical unit. Thesmall-volume flow containing a high concentration of metal salts and/ormetal hydroxides is preferably heated before being led to theelectrochemical unit, whereby the required heat may be withdrawn fromthe heat-containing flow. The required heat may also be provided byusing a heat source.

In the electrochemical unit, the small-volume flow containing a highconcentration of metal salts and/or metal hydroxides is separated into aflow containing one or more metals and a flow containing a lowconcentration of metal salts and/or metal hydroxides.

It is preferred to lead back part of the flow containing a lowconcentration of metal salts and/or metal hydroxides to theconcentration unit, whilst another part is led back to theelectrochemical unit. More in particular, only a small part of the flowis led back to the concentration unit, whilst the larger part of saidflow is led back to the electrochemical unit. According to theinvention, it is generally not necessary to heat the flow containing alow concentration of metal salts and/or metal hydroxides that is ledback to the electrochemical unit. If it should be necessary to heat saidflow, however, this will be possible, of course, for example by mixingsaid flow with the small-volume flow containing a high concentration ofmetal salts and/or metal hydroxides prior to heating the latter flow.

The method according to the invention may be carried out in batches orin a continuous process. It is preferred to carry out the method in acontinuous process.

The invention furthermore relates to a device for recovering metals froma metal-containing flow, which device comprises a dissolving unit, aconcentration unit, and an electrochemical unit.

The concentration unit preferably comprises a heat exchange plant and asettling unit. Said heat exchange plant is preferably provided fortransferring heat from the solution of the metal salts and/or metalhydroxides, via a heat-containing flow, to another process flow that isto be heated, preferably the flow from the concentration unit, whichcontains a low concentration of metal salts and/or metal hydroxides,which flow is preferably led back to the dissolving unit. It ispreferred to provide a settling unit for separating metals from thesolution which has been cooled down in the heat exchange plant, wherebyforming a small-volume flow containing a high concentration of metalsalts and/or metal hydroxides and a large-volume flow containing a lowconcentration of metal salts and/or metal hydroxides are formed.

The heat exchange plant preferably comprises one or more evaporationunits for evaporating the solvent or the solvents which are present inthe solution, and one or more condensation units for condensing thevapour-phase solvent or solvents.

According to the invention, the evaporation unit preferably operates atlow pressure, and the condensation unit preferably operates at highpressure. The combination of evaporation unit and condensation unit, orthe combinations of various evaporation units and condensation unitspreferably operate via the socalled heat pump principle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic embodiment of the device according to theinvention.

FIG. 2 shows a preferred embodiment of the device according to theinvention.

FIG. 3 is a diagrammatic, cross-sectional view of the electrochemicalcell.

FIG. 4 is a diagrammatic, longitudinal sectional view of theelectrochemical cell.

FIG. 1 shows a diagrammatic embodiment of the device according to theinvention. The numerals in the figure refer to the following components:

1. a dissolving unit

2. a concentration unit

3. a heat exchange unit

4. a settling unit

5. an electrochemical unit

6. a heating unit

7. a metal-containing flow

8. a flow of solvent(s), which may contain (an) additive(s)

9. a solution containing metal salts and/or metal hydroxides

10. a vapour and gas flow

11. a heat-containing flow

12. a relatively cold, condensed flow

13. a large-volume flow containing a low concentration of metal saltsand/or metal hydroxides

14. a small-volume flow containing a high concentration of metal saltsand/or metal hydroxides

15. a flow containing a low concentration of metal salts and/or metalhydroxides

16. a flow of recovered metals

17. a flow containing a low concentration of metal salts and/or metalhydroxides, which is led back to the electrochemical unit

18. a flow containing a low concentration of metal salts and/or metalhydroxides, which is led back to the concentration unit

The method and the device according to the invention are in particularsuitable for recovering metals from electric and electronic products,for example printed circuit boards, from galvanised steel from electricappliances, from fly ash from incineration plants and metal meltingplants, from sludge and filter cake from flue gas and waste waterprocessing plants and from galvanised scrap.

According to one preferred embodiment, the combination of concentrationunit, settling unit and heat exchange unit consists of a settling basin,which is on the upper side provided with an evaporation unit, whichoperates at low pressure, and with a vapour compressor. The heat of thesolution of metal salts and/or metal hydroxides is transferred, via theevaporation unit operating at low pressure and the vapour compressor,wherein the solvent or solvents function as a heat-transferring mediumor as heat transferring media, to the large-volume flow containing a lowconcentration of metal salts and/or metal hydroxides, which is led backto the dissolving unit via the overflow of the settling basin before thesolution of metal salts and/or metal hydroxides is carried back to thesettling basin. This preferred embodiment is diagrammatically shown inFIG. 2, wherein (21) is the settling basin, (22) is the evaporation unitoperating at low pressure, (23) is the evaporation compressor, (24) isthe solution of metal salts and/or metal hydroxides, (25) is thesmall-volume flow containing a high concentration of metal salts and/ormetal hydroxides, (25) is the large-volume flow containing a lowconcentration of metal salts and/or metal hydroxides, (27) is theheat-containing flow and (28) is the relatively cold, condensed flow.

According to the invention, the electrochemical unit preferablycomprises an electrochemical cell. A conventional electrochemical cellcomprises an electrochemical vessel, which accommodates a series ofelectrodes, wherein said electrodes alternately function as anodes andcathodes. When metals are being recovered by means of electrochemicalconversion, the metal or metals are deposited on the cathodes, andconsequently the cathodes must be removed from the electrochemicalvessel at regular intervals in order to remove the metal that hasdeposited on the cathodes. The electrochemical cell according to theinvention, however, comprises electrodes, in particular cathodes, ofsuch a type that they need not be removed any more for removing themetal that has deposited thereon. Consequently, the electrochemical cellaccording to the invention is preferably provided with anelectrochemical vessel, which accommodates at least one electrode whichis rotatable about a rotary shaft, preferably a rotatable cathode.

The cathode may have any shape that is suitable, of course, for examplecircular, oval or rectangular. According to the invention, the cathodeis preferably substantially circular.

The cathode is provided with a scraping device, which removes metalsthat have deposited on the cathode by scraping off said metals. Thescraping device comprises a scraping element and a conveyor, whichextends outside the electrochemical vessel. The conveyor cooperates withthe scraping element in such a manner that material which is scraped offthe rotatable cathode is transported to a location outside theelectrochemical cell, where it is subsequently discharged from theelectrochemical vessel through an outlet opening.

The conveyor is preferably made of a chemically inert and wear-resistantmaterial, it may be disposed in an electrically neutral manner or in ananodically or cathodically protected manner.

The conveyor preferably comprises one or more conveyor screws or wormwheels, which are oriented in radial direction with their central axiswith respect to the rotary shaft of the cathode. A housing is presentalong the conveyor screw(s), which housing bounds a transport chambertogether with the surface of the cathode which is being scraped. It willbe apparent that if the ends of the housing are made of a hard material,for example stainless steel, said ends cannot come into direct contactwith the cathode surface in such a manner that this would result in wearof the cathode. Consequently, the ends of the housing are preferablymade of a flexible and/or relatively soft material, for example a brushor a comb made of an inert plastic material, so that the housing,together with the cathode surface, forms a transport chamber via saidflexible and/or relatively soft ends, in such a manner that thescraped-off material is carried substantially outside, and preferablycompletely outside, the electrochemical cell. Furthermore it will beapparent that the housing must enclose the conveyor screw(s) completelyoutside the circumference of the cathode, since scraped-off materialwill otherwise land in the electrochemical cell again.

The scraping element and the conveyor screw may be disposed in anelectrically insulated manner, if necessary, and be anodically orcathodically protected. The cutting edge of the scraping element ispreferably made of a wear-resistant, chemically resistant material, andit can be adapted to the type of metal deposit that is to be scrapedoff.

The scraping element is preferably made up of the conveyor screw. Theadvantage of this is that the cathode surface is being scraped by therotating movement of the conveyor screw, whilst the scraped-off materialis at the same time carried to a location outside the electrochemicalcell.

The side of the scraping element which comes into contact with thematerial deposited on the cathode, or with the cathode as such, may besharp or be provided with teeth or the a like, in a manner which makesit easier to scrape off the deposited material.

According to one preferred embodiment of the invention, theabove-described scraping element is present on either side of thecathode, so that scraping will take place on either side of the cathode.The clearance between the cutting edge of the scraping element and thecathode is adjustable.

The rotation of the cathode and the rotation of the scraping device areeffected via independent drive units, which may be present outside theelectrochemical cell, whereby the rotation of the cathode is accordingto the invention preferably effected via a drive unit which is presentoutside the electrochemical cell. Both the cathode and the scrapingelement can rotate continuously or intermittently at a rotational speedand a time interval which can adjusted as desired.

The scraping device can be disposed at varying angles to the cathodesurface. According to the invention, the selection of the angle betweenthe central axis of the scraping device and the perpendicular throughthe rotary shaft, at the location where the electrode is connected tothe rotary shaft, is a design matter.

The supply of current to the cathode takes place by means of powersupply means, for example sliding contacts such as carbon brushes,whereby said power supply means work outside the electrochemical cell,via the rotary shaft of the cathode.

One preferably of the electrochemical cell according to the inventionwill now be explained in more detail with reference to FIGS. 3 and 4.

FIGS. 3 is a diagrammatic, cross-sectional view, and FIG. 4 is adiagrammatic, longitudinal sectional view of the electrochemical cell.

The electrochemical cell is provided with an electrochemical vessel (1),which accommodates a cathode (2) and an anode (3). The cathode (2)rotates about a rotary shaft (4). The cathode (2) is provided with adevice (5) which scrapes the cathode and which also transport thescraped-off material to a location outside the electrochemical cell.Device (5) preferably comprises two conveyor screws or worm wheels (6),which each act on one side of the cathode. A housing (7) surrounds thedevice (5), which housing, together with the cathode surface, forms atransport chamber, whereby the housing (7) entirely encloses the device(5) outside the circumference of the cathode. The electrochemical cellis furthermore provided with an outlet (8), through which thescraped-off material can be discharged.

The rotary shaft (4) of the cathode is driven via a variable drive unit.The rotary shaft (4) is furthermore provided with one or more powersupply means (10), for example sliding contacts, such as carbon brushes.

Furthermore, one or more membranes (11) may be present in theelectrochemical cell, which membranes separate the anode space and thecathode space from each other, which is for example necessary whenselective transport of ions is desired. Suitable membranes are forexample made of ceramic materials or plastic materials, for exampleperfluorosulphonate polymers, perfluorocarboxylate polymers orcombinations of such polymers, whereby membranes of plastic material maybe reinforced with materials such as polytetrafluoroethylene fabrics.

The electrochemical cell is preferably provided with a gastight cover(12) for the purpose of providing a controlled vapour discharge,protecting the environment and the working area and preventing loss ofsolvent(s), reaction products and heat. The cover may also be providedwith one or more ventilating openings (13).

What is claimed is:
 1. A method for recovering metals from ametal-containing flow, wherein: a. the metal-containing flow and asolvent are supplied to a dissolving unit, whereby a metal-containingsolution containing metal salts and/or metal hydroxides is formed; b.the metal-containing solution is then supplied to a concentration unit;c. the metal-containing solution is separated in the concentration unitinto a small-volume flow containing a high concentration of metal saltsand/or metal hydroxides, and a large-volume flow containing a lowconcentration of metal salts and/or metal hydroxides; d. thesmall-volume flow containing a high concentration of metal salts and/ormetal hydroxides is supplied to an electrochemical unit, whereas thelarge-volume flow containing a low concentration of metal salts and/ormetal hydroxides is communicated back to the dissolving unit; and e. thesmall-volume flow containing a high concentration of metal salts and/ormetal hydroxides is separated in said electrochemical unit into a flowcontaining one or more metals, and a flow containing a low concentrationof metal salts and/or metal hydroxides, said latter flow beingcommunicated back to the electrochemical unit.
 2. A method according toclaim 1, wherein during step c said solution (9) is cooled down, wherebya heat-containing flow (11) and a cooled-down solution (12) are formed.3. A method according to claim 2, wherein said heat-containing flow (11)substantially consists of the solvent or the solvents (8) from thesolution (9).
 4. A method according to claim 2, wherein saidheat-containing flow (11) is led to a heat exchange unit (3).
 5. Amethod according to claim 1, wherein during step (c) said solution iscarried to a settling unit.
 6. A method according to claim 1, whereinsaid large-volume flow (13) containing a low concentration of metalsalts and/or metal hydroxides is heated (6).
 7. A method according toany of the preceding claims 1-6, wherein said small-volume flow (14)containing a high concentration of metal salts and/or metal hydroxidesis heated (6).
 8. A method according to claim 1, wherein during step (e)part of said flow containing a low concentration of metal salts and/ormetal hydroxides is communicated to the electrochemical unit.
 9. Amethod according to claim 1, wherein during step (e) part of said flowcontaining a low concentration of metal salts and/or metal hydroxides iscommunicated to the concentration unit.
 10. A metal according to claim1, wherein the method is carried out in a continuous process.
 11. Adevice for carrying out a method according to claim 1, the devicecomprising: (a) a dissolving unit for mixing a metal-containing flow anda solvent so that a metal-containing solution containing metal saltsand/or metal hydroxides is formed; (b) a concentration unit connectedwith the dissolving unit, seen in the direction of the flow, forseparating the metal-containing solution into a small-volume flowcontaining a high concentration of metal salts and/or metal hydroxides,and a large-volume flow containing a low concentration of metal saltsand/or metal hydroxides; (c) an electrochemical unit connected with theconcentration unit, seen in the direction of the flow, for separatingthe small-volume flow containing a high concentration of metal saltsand/or metal hydroxides into a flow containing one or more metals, and aflow containing a low concentration of metal salts and/or metalhydroxides, (d) said concentration unit being placed in-between thedissolving unit and the electro chemical unit, wherein said electrochemical unit is provided with a conduit for leading the flow containinga low concentration of metal salts and/or hydroxides to theconcentration unit, and wherein said concentration unit has a conduitfor leading the flow containing a low concentration of metal saltsand/or hydroxides to the dissolving unit.
 12. A device according toclaim 11, wherein said concentration unit (2) comprises a heat exchangeunit (3) and a settling unit (4).
 13. A device according to claim 12,wherein said heat exchange unit (3) includes one or more evaporationunits (22) and one or more condensation units.
 14. A device according toclaim 13, wherein said evaporation unit (22) operates at low pressure.15. A device according to claim 13, wherein said condensation unitoperates at high pressure.
 16. A device according to claim 11, whereinsaid electrochemical unit comprises an electrochemical cell.
 17. Adevice according to claim 16, wherein the electrochemical cell comprisesan electrochemical vessel (1) wherein at least one electrode (2), whichis rotatable about a rotary shaft (4), is accommodated in saidelectrochemical vessel (1), which electrode (2) is provided with ascraping device (5), which extends in radial direction with respect tosaid rotary shaft (4), which scraping device (5) comprises a scrapingelement and a conveyor (6) which cooperates therewith, which conveyor(6) extends outside the electrochemical vessel (1), wherein the conveyor(6) transports the material that has been scraped off the electrode (2)to a location outside the electrochemical vessel (1), and whichcomprises at least one conveyor screw, whose central axis extends inradial direction with respect to said rotary shaft (4), as well as ahousing (7) extending along the conveyor screw, which housing bounds atransport chamber together with the surface of the electrode (2), andwhich encloses said conveyor screw outside the circumference of saidelectrode (2).
 18. A device according to claim 17, wherein said conveyor(6) forms said scraping element.
 19. A device according to claim 17,wherein said scraping device (5) is present on either side of theelectrode (2).
 20. A device according to claim 17, wherein the driveunit for providing rotation of the scraping element operatesindependently of the rotation of the electrode.
 21. A device accordingto claim 17, wherein the angle between the central axis of said scrapingdevice and the perpendicular through the rotary shaft, at the locationwhere the electrode is connected to the rotary shaft, is adjustable. 22.A device according to claim 17, wherein the cell is provided with agastight cover.